1. The Field of the Invention
The present invention relates to an optical glass, the production of such a glass, the use of such a glass, optical elements comprising the optical glass or preforms of such optical elements, and optical parts or components comprising such optical elements.
2. The Related Art
Conventional optical glasses of the optical position claimed here (extreme hard flint and lanthanum hard flint position) generally contain PbO in order to achieve the desired optical properties, i.e. preferably a refractive index nd of 1.82≦nd≦2.00 and/or an Abbe number vd of 18≦vd≦28, but particularly in order to achieve the high refractive index. These glasses are therefore chemically not very stable.
As2O3 is furthermore often used as a fining agent. Since the glass components PbO and As2O3 have become regarded as environmentally unfriendly in recent years, most manufacturers of optical instruments and products tend to use lead-free and arsenic-free glasses in preference. For use in high price products, glasses with increased chemical stabilities are also constantly gaining importance.
Known lead-free glasses of the hard flint or lanthanum hard flint position, with a high refractive index and a low Abbe number, generally contain large amounts of TiO2 in a silicate matrix, which leads to extreme crystallization instabilities and therefore to glasses which are often not workable in a secondary molding step. Also glasses containing TiO2 are very difficult to process mechanically.
Instead of the hitherto customary machining of optical components from glass in block or ingot form, production methods in which direct pressings, i.e. precision-pressed optical components and/or preforms for re-pressing which are as close as possible to the final contour, so-called “precision gobs”, can be obtained directly at the end of melting the glass, have recently been gaining importance. “Precision gobs” generally means preferably fully fire-polished, semifree- or free-formed glass portions, which can be obtained via various production methods.
For this reason the need for “short” glasses, i.e. for glasses whose viscosity changes very strongly with temperature, has been reported more and more in the context of melting and molding process technology. This method has the processing advantage that it is possible to reduce the molding times, and therefore the mold closure times, in precision molding close to final geometry. In this way on the one hand the throughput is increased, and on the other hand the mold material is spared, which has a very positive effect on the overall production costs. Furthermore, owing to the faster solidification thereby obtained, it is also possible to work glasses with a stronger susceptibility to crystallization than in the case of correspondingly longer glasses, and pre-nucleation, which could be problematic in later secondary molding steps, is avoided or at least drastically reduced.
For the same reason, there is likewise a need for glasses whose temperature-viscosity profile in absolute terms comprises low temperatures in the molding range. Through lower process temperatures, this also contributes to increased mold lifetimes and, through fast stress-free cooling, to low pre-nucleation rates. This also offers a greater range of potentially more cost-effective mold materials, which is significant particularly in precision molding close to final geometry.
The prior art to be considered in view of the present invention is JP 92027180 B (Hoya Corp.), US 2004-053768 A (Alcatel), US 2005-0202952 (Hoya Corp.) and WO 03/062162 (Ohara).
According thereto, it is possible to produce glasses with a similar optical position or comparable chemical composition, although they show significant disadvantages in direct comparison with the glasses according to the present invention:
JP 92027180 B discloses an optical tellurite-phosphate glass, probably of a similar optical position. Because of the obligatory content of network-forming tellurium and lead oxide, the glass however has—besides its high toxicity which is also present in the glass batch and the raw material—a temperature-viscosity profile that prevents its use for precision molding (length of the glass).
US 2004-053768 A discloses glass compositions for Raman active fiber core glass. These glass compositions necessarily have a high silicate content (30-90 Mol %) and preferred embodiments contain only four components. Besides the only exceedingly small obtainable refractive index this glass has very high absolute viscosity and a length that excludes a processing via precision molding. Furthermore glass with a high silicate content and few further components tends to crystallize and is thus difficult to handle in secondary precision molding processes of optical components/parts (lenses, prisms, and others).
US 2005-0202952 discloses a glass system for precision molding, which in contrast to the glass of the present invention has the disadvantage of a higher tendency towards crystallization. The reason for this is based on the obligatory silicate content of up to 4 Mol %. In principle SiO2 (in contrast to Bi2O3 and/or GeO2) leads to solubility problems in phosphate matrices, so that its network forming effect (known from different glass systems) is converted into the opposite effect and crystallization, especially in secondary precision molding processes, but also in the melt during primary precision molding, is promoted. Furthermore a light scattering, colloidal precursor is produced already before crystallisation, which precursor remarkably reduces at least the transmission of the glass. Additionally SiO2 decreases the achievable refractive index and dispersion.
WO 03/062162 discloses a niobium barium phosphate glass. This glass is stabilized against devitrification according to the mentioned examples using at least SiO2 and/or B2O3. The glass of WO 03/062162 has the following disadvantages in contrast to the glass according to the present invention, which need to be taken into account. If SiO2 is used in a phosphate glass system, the disadvantages regarding crystallization, optical position and transmission already discussed in US 2005-0202952 result. The use of B2O3 is forbidden in high quality optical phosphate glass because of crucible dwelling time: in connection with the glass that is already very reactive to flameproof materials, because of their phosphate matrix, this effect is even increased drastically with the use of B2O3 (notably in combination with Li2O) and the crucible dwelling time is extremely shortened. The additional colloidal transfer of the crucible material in the melt strongly decreases the transmission over the whole wavelength range. If the material of the crucible is platinum, a platinum alloy or a related alloy, dissolved contents of metal ions further will decrease the transmission through specific absorption. This effect is often observed at the “blue edge” of the spectral range, which is sensitive anyway. Moreover the obligatory B2O3 content restricts the achievable optical position of the glass to low refractive indexes and dispersions. For the technical glasses claimed according to this state of the art especially refractive index and transmission lowering effects are of no importance, quite contrary to the importance such effects have on the optical glasses according to the present invention.